1. Field of the Invention
This invention relates generally to honeycomb structures formed of ceramic materials. More specifically, this invention relates to the forming of ceramic materials into thin wall honeycomb structures by extrusion.
2. Description of the Prior Art
The term honeycomb structures is used generally to describe a thin walled body having a series of regularly or irregularly shaped parallel channels that extend continuously over the length of the body and are separated by wall elements that give the body its structure. The cross-section of each channel may vary from channel to channel but usually will have a regular geometric shape. These honeycomb structures find use in regenerators, heat exchange equipment, filters, and as catalyst carriers. The use of such carriers is also well known in the treatment of automotive exhaust gases where the carriers are typically treated with a wash coat of catalytic material.
Ceramic honeycombs have been formed by extrusion methods with fairly good success. The extrusion method uses a hydraulic ram to push the extrudable material into a series of feed passages which communicate with a discharge area. The discharge area has a series of projections that displace the extrudable material from the sections that will eventually correspond to the channels and define a series of gaps which shape the extrudable material into the walls of the honeycomb structure. It has become common practice to extrude honeycombs having channel densities of from 100 to 200 channels per square inch upon extrusion, and 200 to 400 channels per square inch after shrinkage of the extrudable material during curing. Typically the wall thicknesses between the channels of the honeycomb structure will vary between 0.002 inches and 0.050 inches.
Methods and apparatus for forming extruded honeycomb structures are further described in U.S. Pat. Nos. 3,790,654, 3,905,743, 3,824,196 and 4,550,005.
U.S. Pat. Nos. 3,905,743 and 3,790,654 issued to Bagley describe a method for forming a thin walled honeycomb extrusion that uses a die having feed passages and intersecting feed slots. Bagley claims and primarily teaches aligning the feed passages to communicate directly with the interconnections or intersections between a series of orthogonal slots. U.S. Pat. No. 3,824,196 issued to Benbow et al., describes a method of making a thin walled honeycomb structure by passing a plastic material through a die having a series of feed passages that again intersect and communicate directly with intersecting points in a series of orthogonal slots that define the shape of the extrusion. Benbow also teaches that the feed passages should have a greater cross-sectional area than the transverse cross-sectional area of the discharge slots in order to provide sufficient material for filing the discharge slots.
U.S. Pat. No. 4,550,005 issued to Kato teaches a method of extruding a honeycomb structure having walls of varied thickness and a die for use therein. The die and the method of Kato use feed passages having a hydraulic diameter that varies in relation to the walled portion being formed thereby. The feed passages are varied such that feed passageways associated with a thin walled portion have a relatively large hydraulic diameter, and feed passageways associated with thick wall portions have a relatively small hydraulic diameter.
As the above prior art demonstrates it has been believed that it is necessary to provide feed passages with a greater cross-sectional area than that of the discharge slots over the die in general or at least in areas where the discharge slots have a low hydraulic diameter. In addition, the tendency to directly feed plastic material into the most open section of the discharge passages and rely on lateral flow to fill more narrow sections forces remixing and mechanical reworking of the material to occur over a region of reduced cross-section. Therefore methods for extruding honeycomb structures have not been arranged to maximize the strength of the honeycomb structure. Furthermore, providing additional area for the feed slots and locating and aligning the feed slots to maximize flow into the discharge slots, has complicated the design and fabrication of dies and led to die designs that have less than optimal structural integrity.